I think i was bite in my arm is swollen, red hard to the touch, heat. what bit me?

Rarely will the appearance or location of a bite reveal what did it. If you catch the bug in the act, you can, otherwise there is no way to know.

For insect bites, circumstances are many times pointing towards a likely insect. Indoors/outdoors exposure, common insects in the area, time of the day, etc as the initial inflammation is most times non-specific. Majority of insect bites do not need specific treatment as they heal well, some do (specifically ticks, spiders sometimes).

What, if anything, bit you is not important. You have spreading bacterial cellulitis (a skin infection), and the hardened areas will develop into abscesses (pus-filled cavities that need to drain). See a doctor or go to the Emergency Room TODAY for antibiotics and possible surgical drainage. You will also need a tetanus shot if it has been more than five years. Infections can be unforgiving!